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Compression ratio is another factor that affects performance and engine power. The only thing that creates engine power is the size of the explosion of fuel and air in the combustion chambers and more compression creates a bigger the explosion. Once compression ratios start to get bigger, regular fuel can start to ignite before the spark plugs ignites and this can cause problems. So to prevent that from happening, manufacturers recommend or require higher octane fuel (can withstand a greater compression before igniting) on engines that have higher compression ratios.
The 6.4L SRT engine has a compression ratio of 10.9:1, which is significantly higher than the 3.6L engine’s compression ratio of 10.2:1 or the 5.7L HEMI engines compression ratio of 10.5:1. Premium gas (highest octane) is recommended for the 6.4L SRT engine, whereas midgrade gas is recommended for the 5.7L HEMI and regular gas is recommended for the 3.6L engine.
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Truck version of the 6.4 has a lower static CR of 10
From
https://www.universitydodge.com/dodge-gearhead-6-4l-srt-engine-performance-specs/
